Thermally induced fracturing in hot dry rock environments - Laboratory studies

Abstract

In enhanced geothermal system (EGS) wells, where strong thermal gradients exist, hydraulic fracturing is the apparent driver for creating permeability in the reservoirs. Thermal effects, however, have been observed to accompany hydraulic loading and may further enhance the permeability of the reservoir. The mechanism of thermal fracturing in downhole conditions, however, is not well understood, limiting the efficient engineering of thermal effects on EGS stimulation. This laboratory study examines the behavior of thermal shock stimulation and temperature propagation by flowing room-temperature water through a borehole of a hot specimen block without confining stresses or borehole hydraulic loading. This condition isolates the effect of thermal shock from hydraulic pressure and confining stress. We monitored temperatures across the inside of the specimens and assessed fractures by visual inspection, bubble leakage, pressure decay, and acoustics. Thermal loading resulted in an enhancement of permeability by induced macro/microcracks. The profiles of borehole pressure decay obtained before and after each stage of stimulation show increased permeability of the treated specimens. The acoustic measurements indicate the extent of fractures, and the bubble leakage tests on the specimen surface visually indicate localized permeation paths created by newly created cracks. The maximum thermal gradient was achieved near the borehole surface at the start of the water flow. As the flow continued, the magnitude of the thermal gradient near the borehole decreased while that of the thermal gradient away from the borehole increased. Thermally driven fractures, initiated from the borehole walls, propagated perpendicular to the borehole surfaces. These “seed” fractures created during thermal stimulation, although they might be suppressed under very high stress, help reduce the levels of breakdown pressure in concurrent or subsequent pressure-based fracturing methods.